The stators of many dynamo-electric machines, for example motors and electric generators, comprise a toroidal metal core and a plurality of copper or aluminum wire windings, passing through the core, suitable for generating a magnetic field necessary for the operation of the machine. For improving the machine efficiency and for avoiding that a winding wire interferes with the rotor, the windings are generally wrapped, in particular the parts that project out of the core, by means of a cotton or synthetic material cord, for example polyester.
In order to effect the lacing of the stators in an automatic way, suitable automatic lacing stations have been developed. For instance U.S. Pat. No. 3,685,470 describes a machine for the lacing of stators that effects a type of lacing known as “diamond stitch” or “honeycomb.” Using this machine however, when the lacing is finished, the operator must manually knot the initial and final terminals of the cord, and this operation affects considerably the production costs and times.
In order to obviate this drawback knotting stations have been developed that provide, when the stator gets out of the lacing station, to knot together the initial and final terminals of the cord. The addition of a station in the production line affects however considerably the costs of the plant and the maintenance costs and increases the probabilities of a machine stop due to failures on the production line.
U.S. Pat. No. 3,983,826 discloses a device that locks the starting lacing terminal of the cord and subsequently effects a knot on the end lacing terminal. The knot made by such device is however a rather complex knot that lengthens considerably the working cycle of each single piece.
U.S. Pat. No. 4,614,161 discloses a device that effects automatically the lacing of a stator coil, using a diamond stitch, and, at the end of the lacing phase, effects a knot on the end terminal of the cord. The effectiveness of the knot is assured by the melting of the free end of the cord (a cord made of polyester or other polymeric material), which is contemporaneously cut by means of an incandescent wire. Such device has however some drawbacks: the effectiveness of the knot is in fact strongly conditioned by the dimensions of the melted “little ball” formed during the cut, which can vary depending on many main factors (quality of the cord, temperature of the incandescent wire, etc.). Moreover, at each cut, a superfluous cord portion, known as “scrap”, is produced which must be immediately removed and which represents an economical and ecological waste.
U.S. Pat. No. 4,558,894 discloses a device for tying or lacing a bundle of electrical wires in which, upon formation of a predetermined number of cord loops around the bundle, the lacing process is automatically stopped and a number of additional loops is laid in a chain-like formation on one side of the bundle. Once the desired number of additional loops has been laid, the cord is pulled tight to form a knot and is cut to allow starting a new lacing phase. Practical experience has shown that a knot made in such manner is not reliable and is prone to become readily untied.